The present invention relates to an apparatus for cancelling noise and improving details of an image signal.
FIG. 1 shows a conventional noise canceller and FIGS. 2A through 2D show waveform diagrams for explaining an operation of the FIG. 1 apparatus.
As shown in FIG. 2A, noise contained in an image signal is mostly formed of high-frequency components. An image signal including such noise is applied to a high-pass filter 1 and an adder 3. High-pass filter 1 filters the input image signal shown in FIG. 2A and extracts only high-frequency components as shown in FIG. 2B. A limiter 2 outputs a signal having an amplitude equal to or less than a predetermined value among the high-frequency components supplied from high-pass filter 1 and then supplies the signal to adder 3. Here, the predetermined value is determined based on a fact that a noise signal usually has a small amplitude. Thus, the signal output from limiter 2 becomes a noise signal contained in the high-frequency components, which is shown in FIG. 2C. Adder 3 adds the FIG. 2A signal and a signal inverting the output of limiter 2, to generate an image signal shown in FIG. 2D. However, as can be seen from an image signal shown in FIG. 2D which is changed from the FIG. 2A waveform so that edges of the image signal are rounded, the FIG. 1 apparatus removes even detail components such as an edge as well as the noise contained in the high-frequency components of the image signal. The image signal from which the noise is cancelled by such a conventional noise canceller lowers resolution of the image signal to be displayed.